Mach-Zehnder modulators (MZMs) are widely used in fiber-optic communication systems. In a coherent optical fiber communication system, two parallel MZMs and a 90-degree phase shifter are usually used to form an in-phase quadrature (IQ) Mach-Zehnder modulator (IQ-MZM) to respectively modulate in-phase and quadrature components of a complex signal.
FIG. 1 is a schematic diagram of a structure of an optical modulator in a polarization multiplexing system, in which a case of dual polarization states (denoted by h and v) is shown. A structure of an IQ-MZM is as shown by dotted lines in FIG. 1. As shown in FIG. 1, s(t) is a drive signal of the MZM, and D is an optimal direct current bias voltage. Whatever it is used for single polarization modulation or for dual polarization modulation (for example, as shown in FIG. 1), each MZM in the IQ-MZM usually operates independently at respective optimal direct current bias point. However, due to environmental changes and device aging, the direct current bias of the MZM will drift. As shown in FIG. 1, ε denotes the direct current bias induced by the drift.
The direct current bias induced by the drift may affect a modulation performance, thereby bringing about damage to system performance. Especially, with gradual upgrade of modulation formats, such as 16 quadrature amplitude modulation (QAM), 32 QAM, or even formats of higher orders, the system is increasingly sensitive to the MZM direct current drift.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.
Documents advantageous to understanding of this disclosure and conventional technologies are listed below, which are incorporated herein by reference, as they are fully described in this text.    [1] U.S. Pat. No. 6,539,038, James Allan Wilkerson, et al. “Reference frequency quadrature phase-based control of drive level and DC bias of laser modulator”.    [2] Kenro Sekine, et al. “A Novel Bias Control Technique for MZ Modulator with Monitoring Power of Backward Light for Advanced Modulation Formats”, OSA 1-55752-830-6.    [3] Constantinos S. Petrou, et al. “Quadrature Imbalance Compensation for PDM QPSKCoherent Optical Systems”, IEEE Photon. Technol. Lett., vol. 21, no. 24, pp. 1876-1878, Dec. 15, 2008    [4] L. Li et al., “Wide-Range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers”, OFC2008, OWT4.    [5] J. Li et al., “Laser-Linewidth-Tolerant Feed-Forward Carrier Phase Estimator With Reduced Complexity for QAM”, JLT 29, pp. 2358, 2011.    [6] H. Louchet et al., “Improved DSP algorithms for coherent 16-QAM transmission”, ECOC2008, Tu.1. E.6    [7] W Yan et al., “Low Complexity Digital Perturbation Back-propagation”, ECOC2011, Tu.3.A.2.